OBJECTIVES (1) To examine the social and seasonal interactions on reproductive functioning in male muriqui monkeys. (2) To examine the controls of fertility in young females immigrating and emmigrating from the group (3) To evaluate reproductive failure in muriqui females showing sexual activity but not conceiving. RESULTS Fecal testosterone and cortisol levels were analyzed from six wild male muriquis (Brachyteles arachnoides) over a 19-month period at the Esta o Biol-gica de Caratinga in Minas Gerais, Brazil to investigate the hormonal correlates of seasonal sexual behavior and environmental conditions. Group mean testosterone levels based on weekly samples from the six males did not differ between copulatory and noncopulatory periods or between rainy and dry seasons. Cortisol levels did change with copulatory periods, and were significantly higher during the second dry season, when mating continued following an exceptionally heavy rainy season, than during the first dry season, when mating ceased. Males exhibited individual variation in the timing of their hormone shifts relative to their sexual activity, but neither hormone levels nor sexual activity were related to male age. Despite individual differences in the timing of testosterone fluctuations around the onset and offset of the copulat ory season, all males exhibited elevated cortisol concentrations following a slight increase in testosterone at the beginning of the copulatory season. Both the lack of significant changes in testosterone levels with the onset of the rainy and copulatory season and the lack of prebreeding increases in cortisol may be related to the low levels of overt aggression displayed by male muriquis over access to mates. The onset of hormonal activity is currently being monitored in young females who are immigrating from or emmigrating into the group to determine if sexual maturity leads to removal from the group or if sexual immature females leave the group before they become sexually mature and possibly mate with related males. FUTURE DIRECTIONS We plan to examine both male and female muriqui reproductive seasonality simultaneously to better understand the social and environmental factors influencing hormone release. KEY WORDS fertility, social and environmental affects, wild muriqui monkeys, New World primate PUBLICATIONS Ziegler T.E., Strier, K.A. Seasonal responses of fecal cortisol and testosterone levels from free-ranging male muriqui monkeys, Brachyteles arachnoides, American Journal of Primatology 45(2):117, 1998. [A]